1. Field of the Invention
The present invention relates to a technology for estimating, while driving, the internal resistance (IR) of a secondary battery such as a nickel-metal hydride (Ni-MH) battery installed in a pure electric vehicle (PEV), a hybrid electric vehicle (HEV), or the like as a source of motive power for the motor and a source of driving power for various loads and for detecting the state of deterioration and an anomaly of the secondary battery.
2. Description of Related Art
Conventionally, it is well known that the internal resistance of a secondary battery is calculated to detect the state of deterioration or an anomaly of the secondary battery. In HEVs and other electric vehicles, when an output from the engine is greater than the motive power that is necessary for the vehicles to run, the surplus motive power is used to drive the generator so as to charge the secondary battery. Conversely, in HEVs and other electric vehicles, when an output from the engine is small, the electric power of the secondary battery is used to drive the motor so as to make up for the shortage of the output from the engine.
For example, JP 2000-323183A discloses a method for detecting the internal resistance of a secondary battery that is connected to a load consuming or regenerating such electric power. More specifically, the current I, voltage V, and temperature T during charging and discharging are first sampled over a predetermined period of time. Next, the sampled currents I are integrated to obtain the maximum amount of stored charge Qmax and the minimum amount of stored charge Qmin in that period, and furthermore, the difference ΔQ between these stored charges is obtained. Then, when all aspects, i.e., the difference ΔQ in the amount of stored charge, a distribution of the current I, and the temperature T, in that period are appropriate, a linear approximation of the sampled currents I and voltages V is obtained, and the internal resistance is calculated from the slope of the obtained straight line.
In this manner, the internal resistance of the secondary battery installed in HEVs and other electric vehicles is calculated based on the current I and voltage V that are measured as the vehicles are running.
However, even when the secondary battery is normal, the current I and voltage V that are measured during running may vary depending on the state of charging and discharging pattern at the time when the vehicle starts to run or stops running, for example. Moreover, in some cases, a small short-circuit occurs in the secondary battery, resulting in a deterioration in the secondary battery, and thus the voltage V decreases only during discharging. Consequently, it is difficult to obtain a correct value of internal resistance with the method described above.